ksm: cond_resched in unstable tree

KSM needs a cond_resched() for CONFIG_PREEMPT_NONE, in its unbounded
search of the unstable tree. The stable tree cases already have one,
and originally there was one down inside get_user_pages();
but I missed it when I converted to follow_page() instead.

Signed-off-by: Hugh Dickins <[e-mail]>
Acked-by: Izik Eidus <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/ksm.c

···
 		struct rmap_item *tree_rmap_item;
 		int ret;
 
+		cond_resched();
 		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
 		page2[0] = get_mergeable_page(tree_rmap_item);
 		if (!page2[0])